Alcohol dehydration reactions to produce alkenes have been known for a long time. Usually these reactions are performed in presence of solid acid catalysts, the conversion of alcohol being nearly complete. However, in view of the potential downstream applications of olefins, it is of particular importance to limit the amounts of secondary products to gain in process efficiency and to save expensive downstream steps of separation/purification.
It has been observed, in addition to dehydration of alcohol to the corresponding olefin, formation of aldehyde, in particular formation of the corresponding aldehyde, and formation of light products such as H2, CO, CH4. It is supposed that formation of H2 and CO results mainly from degradation of said aldehydes under the conditions of the dehydration reactions. Formation of H2 and CH4 may result from other kind of side reactions. For example, during dehydration of ethanol, formation of acetaldehyde, H2, CO, and CH4 is observed. Similar undesirable secondary by-products can be observed during dehydration of other alcohols. These secondary products lead to lower once-through yield of the corresponding olefin and to important losses of the olefin, in particular in downstream purification section. The formation of these products is still not well understood and solutions provided by prior art to reduce the formation of these secondary products are limited.
WO2011/002699 discloses a process for producing olefins by dehydration of alcohols in reactors under either adiabatic or isothermal conditions. The process comprises reacting under first reaction conditions an aliphatic alcohol, optionally diluted with water, in the presence of a dehydration catalyst to form a first reaction product that includes dialkyl ether and generated water, and further reacting under second reaction conditions the first reaction product in the presence of a dehydration catalyst to form olefin by dehydration of the dialkyl ether. The temperature of the second reaction conditions is at least 10° C. higher than the temperature of the first reaction conditions. In particular, the temperature of the first reaction conditions ranges from 200° C. to 450° C., while the temperature of the second reaction conditions ranges from 250° C. to 500° C., preferably from 400° C. to 450° C. The purpose of the relatively low temperature range in the first reactor is to instigate reaction of the aliphatic alcohol to primarily its corresponding dialkyl ether, which dehydration serves to increase the water content of the first reaction product. The effect of the temperature increase between the first and second reactors is that the amount of dialkyl ether may be progressively reduced as dehydration is carried to or toward completion, to form the final desired olefin, and the reduction in starting diluent water with the alcohol feed means that there is a minimum of corresponding aldehyde formed. There is no mention of other by-products such as H2, CO or CH4.
U.S. Pat. No. 4,232,179 relates to a process for preparing ethene by dehydrating ethyl alcohol in the presence of catalysts using adiabatic reactors at high temperature. In that process, the necessary heat to maintain the temperature of the catalyst bed at levels compatible with the desired conversion is supplied by the simultaneous introduction of the feed and a sensible heat carrying fluid, which may be selected from, for example, a part of the effluent from the reactor used as a recycle stream, steam supplied by an external source, other adequate fluids for the process, or any combination thereof. The use of diluted ethyl alcohol in the sensible heat carrying fluid stream leads to considerable reduction in the formation of C3 and C4 by-products, as well as in the deposition of coke over the catalyst, these peculiar features leading to highly pure ethene. There is no mention of other by-products such as H2, CO or CH4.
U.S. Pat. No. 4,396,789 relates to a process for the dehydration of ethanol to form ethene in fixed adiabatic reactors containing a dehydration catalyst. The process includes the recycling of unreacted ethanol to the process, feeding the charge to the initial reactor at a pressure of 20 to 40 atm, withdrawing the ethane from the final reactor at a pressure of no less than 18 atm, and passing at least a portion of said reaction effluent to cryogenic purification with further compression. Ethyl alcohol is introduced with steam at a temperature from 400° C. to 520° C. and a pressure from 20 to 40 atm. Subsequent washing and purification steps permit to obtain a high purity ethene. There is no mention of by-products such as H2, CO or CH4.
WO2011/161045 relates to the dehydration of alcohols on acidic catalysts to make the corresponding olefins. The unselective reactions that need to be suppressed are (i) altering in number of carbon atoms compared to the alcohol through oligomerisation and cracking reactions and (ii) the formation of paraffins and aromatics or coke through hydrogen-transfer reactions. In that process, the activity and selectivity of alcohol dehydration catalyst is adjusted by poisoining the unselective acid sites of the catalyst by spiking the feed with a neutralizing agent while keeping active the selective acidic sites of the catalyst. The neutralizing agent can be chosen from basic compounds: ammonia, organic ammonium salts, hydrazine, nitriles, amines, (including pyridines, pyrrols, pyrrolydones and pyrrolidines), amides, imines, di-imines, imides, cyanates, isocyanates, nitrites and nitroso compounds, aldehydes, ketones, carboxylic esters, and their thio-compounds (thiols, sulphides, disulfides). Secondary light products as H2, CO, CH4 are not mentioned. The spiking is used to moderate the excess of catalyst acidity.
U.S. Pat. No. 4,847,223 discloses the deposition of trifluoromethanesulfonic acid (TFA) onto an acid-form pentasil zeolite to convert ethanol into ethylene. Such acid is coated on the catalyst, HZSM-5 being exemplified. The TFA stays on the catalyst and is not part of the stream of ethanol to be dehydrated.
U.S. Pat. No. 4,423,270 discloses the use of a substituted phosphoric acid as catalyst for dehydration of an ethanol into ethylene. The acid is absorbed on a porous granular support. The acid is therefore not part of the stream of ethanol to be dehydrated.
Nieskens et al. in Industrial & Engineering Chemistry Research 2014, 53, 10892-10898 discloses the addition of a methyl acetate compound at the inlet of the methanol to olefin (MTO) and dehydration reactor. The working conditions to perform those two reactions simultaneously differs from performing the dehydration only as some heat produced by the MTO reactions can readily be used for the dehydration reaction.
E 2 108 636 discloses the used of CO2 as inert component able to bring heat to the dehydration reaction. This document is not concerned about the selectivity of the dehydration reaction.
WO 2013/017496 discloses the dehydration of ethanol over a P-ZSM-5 catalyst. This application discloses a particular catalyst composition tested for the dehydration of the ethanol. However this application is not concerned about the amount of H2, CO, CH4 produced by the catalyst and the way to limit the formation of such by-products.
Prior arts teach us how to improve selectivity in the dehydration products by poisoning the unselective acid sites on the catalyst and inhibit cracking and oligomerization of the alkenes. However, formation of H2, CO, CH4 by-products typically occurs via a different route relative to the acid catalyzed reaction pathway. So, an object of the present invention is to reduce formation of secondary by-products, in particular formation of aldehydes and of light products such as H2, CO, CH4. In particular, the by-products H2, CO, CH4 lead to purification problems downstream the dehydration units as cryogenic temperature are needed in order the separate them from the other components. There is therefore a need for limiting the production of such by-products as much as possible.
A convenient solution has been discovered to reduce the amount of secondary products, light products (H2, CO, CH4) and aldehydes, and to improve the yield of olefin in alcohol dehydration reactions by adding with the alcohol feed organic acid(s).
Without willing to be bound by any theory, it is supposed that metallic sites, which are able to promote the formation of the aldehyde, in particular the corresponding aldehyde, may catalyze side reactions leading to the formation of these secondary by-products. In particular, it is believed that a transformation of the alcohol into the corresponding aldehyde first occurs and is followed by formation of light products such as H2, CO, by degradation of this corresponding aldehyde into lighter products, for example by decarbonylation of the aldehyde. Formation of CH4, but also of some H2, may result from other side reactions, probably catalyzed by the same sites.
The origin of these metallic sites is still uncertain and may be various. They are thought to be present on metallic internal surface of the dehydration unit, in particular metallic internal surface in contact with the feed before the entry of the feed in the reaction zone or in the reaction zone. It is also thought that the sites may also be present on catalyst, either as part of the catalyst or coming from degradation by corrosion of these metallic internal surfaces in contact with the feed. It is also believed that regeneration of the catalyst may lead to an activation of the sites responsible for the formation of the above mentioned undesirable by-products.
Without willing to be bound by any theory, it is supposed that organic acid(s) poison, probably via a stronger adsorption relative to the alcohols, the sites on which these secondary products are formed. It seems that organic compound with an acidic character can selectively poisons the most active sites, which dramatically reduces side reactions and improves the yield of olefin.